From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 32BD63939C9; Wed, 18 Mar 2026 08:53:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773823991; cv=none; b=MwyyOvNvp8RVP02JbCuuGyVxbvsw8FA9g2XXLWba+W+5sxkPFAo7wTocnrJJyBJviMIqw/4hTS87+M7/vufq+6p5nYOlno2l0sF2jEqQ10wfMKsBzZ20D2Xvecx1Bpyps+9JJQ7RVe19Qu3sOj3wmO+Wuu6jRh4zKLs7BiwEKjY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773823991; c=relaxed/simple; bh=hlsD8LO9ExQzsDMc9/GoTSMCxjp+n8FGdWB39l3oGEE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ef9HxwcOeC25qiH690Ox215eBuG+mNwDByS4OrAfr7eFvJu8tfQS3v1g3VnaI2biBOh1eZCxnQJUmbo1ih00cjHqpiSgkJRkCRhfcr0L0eVZng7xG8Dzg29Yn79gGobui7KTvn8Twr83rLjZBOCXYSVWV2PEwuw/jjeAlOld9ss= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HjGLIYYC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HjGLIYYC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8CFD2C19424; Wed, 18 Mar 2026 08:53:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773823990; bh=hlsD8LO9ExQzsDMc9/GoTSMCxjp+n8FGdWB39l3oGEE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=HjGLIYYCS/Bh9mCxklMCiMmbNHL6fgPurJobIe6qRh9VBaAC/iM7NdFpK2R9gynq0 JQhzfxF20REoqQrpyfhFeIKgnv0s02c5IzdxK4ygAdCZPnCPISsY/+vWed/V5NMn3Q hbh6lnEkSBBDr/edhbOgD3aV8uiHumHo7oBs/RCz47g0qTz0emJX2XRgPJGP9Jpnn6 +z2s8BqjYQF27ck6Sc8NhgizwELcmrtXBe/3WvKOnWqIJ2HCibQOtA8HLzQfglUrKP Qs/FNrBHPOrJxUTZBQsNASlIufisA29OabVDXYNLYn921/v08TvJssXH4ouDSomTon vpqBO4fmTLd8w== Message-ID: <217b154d-e4d1-4824-b2e3-fea82bc44402@kernel.org> Date: Wed, 18 Mar 2026 09:53:05 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: cxl/region.c improvements and DAX/Hotplug plumbing To: Gregory Price Cc: linux-cxl@vger.kernel.org, dan.j.williams@intel.com, dave.jiang@intel.com, jonathan.cameron@huawei.com, alison.schofield@intel.com, ira.weiny@intel.com, dave@stgolabs.net, linux-kernel@vger.kernel.org, kernel-team@meta.com, vishal.l.verma@intel.com, benjamin.cheatham@amd.com, David Rientjes References: <4d66eac3-1a2b-4d9d-8a9a-529a19758439@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 1/23/26 01:28, Gregory Price wrote: > On Thu, Jan 22, 2026 at 11:14:15PM +0100, David Hildenbrand (Red Hat) wrote: >> Some of that (especially the interaction with core-mm) feels like it would >> be a good fit to discuss with he wider MM community in one of the bi-weekly >> mm meeting. (CCing David R.) >> > > There is a Monthly Linux-DAX meeting, and a Monthly Linux-CXL meeting, > obviously this is a lot of cross-attendance. > > Happy to attend additional discussion. I was trying to shore up some of > the cxl-region plumbing aspects before going wider. Oh hey, I found an unanswered mail in my inbox :) Sorry for stumbling over this that late. > >>> - hiding memory blocks? (discussed in last meeting) >> >> What is that about and what was the result of that discussion? :) >> > > It was just a question as to whether memory blocks are still useful > if the intent is to provide a collective hotplug interface. I don't > think there are any real proposals for this, just making note of it. Okay, thanks. > >>> Solution 2: Make a dedicated sysram_region with policy >> >> What kind of region would that be? > > plumbing between regionN and dax_region kobjects > > right now the kobject relationship is: > > region0 <- cxl driver created kobject > └dax_region0 <- default selects IORESOURCE_DAX_KMEM > └dax0.0 <- auto-probes on discovery > > But there is baggage in the existing plumbing: > > 1) dax/cxl.c => hard-coded IORESOURCE_DAX_KMEM for dax_region > 2) dax/bus.c => devdax is probed on discovery w/o manual bind step > 3) cxl/core/region.c => BIOS-configured CXL regions automatically > generate a dax_region, and this auto-creates a dax_kmem device > which is subject to system-wide MHP policy. > > This creates a backwards compatibility headache. Agreed. > > The same auto-plumbing is used in the manual creation path, so: > > echo regionN > cxl/decoder0.0/create_ram_region > /* program decoders */ > echo regionN > cxl/drivers/region/bind > > will pump the whole thing directly into dax_kmem and auto-online > according to system default MHP policy. There's no intermediate > step in which the user can define preferences (unless you add > them as attributes to regionN - which is another option). > > Adding the intermediate object: > > regionN > └sysram_region <- encodes policy like hotplug and dax drv > └dax_regionN <- which would be passed here on creation > └dax0.0 > > lets the cxl-cli command to be more expressive: > `cxl-cli create-region -t ram --driver=sysram` => kmem > `cxl-cli create-region -t ram --driver=dax` => device_dax > > and would change the sysfs pattern to > echo regionN > cxl/decoder0.0/create_ram_region > echo regionN > cxl/drivers/sysram_region/bind > echo online_movable > cxl/devices/dax_regionN/hotplug > echo dax_regionN > cxl/drivers/dax_region/bind > > and gives the user a chance to configure a policy before the region > is pumped all the way through to the endpoint dax driver. Would that still be backwards-compatible? >>> Solution 2: dedicated sysram_region driver w/ or w/o DAX. >>> Can support sparseness w/o DAX (see DCD problem) >>> Could use DAX for tagged DCD regions. >>> Tradeoff: May duplicate some DAX logic. >> >> How would that look like? > > For untagged extents w/o dax: > > sysram_region->nr_range > sysram_region->ranges[0 : nr_range-1] > > Extents in this list would be hotpluggable individually and > could be returned to the DCD device individually > > sysram_region.c code would call hotplug directly, not via dax. > - hence, this duplicates some DAX logic > > The above just prevents needlessly creating dax-indirection for sysram > extents with only one destination: add_memory_driver_managed() > > > For tagged extents: > sysram_region->nr_regions > sysram_region->dax_regions[0 : nr_regions] > > A set of tagged extents would only be hotpluggable as a group > and could only be returned to the DCD as a group. > > it would also expose: dax0.0/uuid <- contains the tag Interesting. > > > from this you get a cli command like > > cxl release-extents regionN [--id=X] [--tag=Y] > > translates to something like > > echo "release" > regionN/sysram_region/extents/[X,Y] > > Something like this. > >>> >>> Solution 4: Prevent non-driver actions from changing state. >>> Also solves hotplug protection problem (see next) >> >> The crucial part is solving what you spelled out in the description: "race >> conditions". Forbidding someone to re-configure system RAM sounds >> unnecessary. >> >> For example, I use it a lot for testing issues with page migration while >> offlining memory from ZONE_MOVABLE. >> > > For most use-cases yes. For something like FAMFS (distributed shared > memory), one system onlining a block as kmem could be potentially > destructive to an entirely separate physical server. Right. But shouldn't we fail this already at the add_memory() stage? Sounds like during onlining is a bit too late. Conceptually, the hotplug as sysram was already wrong for famfs, or am I wrong? > >>> Example: Slow(er) memory >>> Some memory is "just memory", but might be particularly slow and >>> intended for use as a filesystem backend or as only a demotion >>> target. Otherwise its allocated / mapped like any other memory, >>> but it still required isolation so isolated to the demotion path >>> and not a fallback allocation target >> >> That doesn't quite fit the description of N_PRIVATE_MEMORY, though. Or what >> am I missing? > > I suppose we could also explore a per-node fallback policy to accomplish > this - but there was also the LPC talk about trying to deprecate that > entirely. I'm looking forward to that LPC talk! -- Cheers, David